Tissue characterization is of great interest in disease diagnosis and treatment. For example, in atherosclerosis, the composition of an atherosclerotic plaque is directly linked to the vulnerability of the plaque. Despite advances in determining risk factors and treating myocardial infarction, there are few techniques available for imaging the development and/or composition of atherosclerotic plaques, which play a major role in cardiovascular disease.
Macrophages are one of the key components involved in the pathology of atherosclerosis. At a relatively early stage in atherosclerotic development, macrophages are present at the “crime scene”: macrophages are formed from the blood monocytes that enter the arterial wall because of the initial inflammation in the arterial endothelial layer. During the progression of the disease, lipid-laden macrophages play a partial role in the formation of atheroma. In particular, macrophages located in the arterial wall may accumulate low density lipoproteins (LDL) through endocytosis and form foam cells. Foam cells can form lipid pools under the endothelial layer of the arterial wall, which contributes greatly to the vulnerability of a plaque. Macrophage infiltration into the fibrous cap of plaques also accelerates disease progression by causing the release of matrix metalloproteinases (MMPs), which weaken the fibrous cap and make the plaques prone to rupture. Clearly, the distribution and activity of macrophages provide important information on the development of atherosclerotic plaques. Therefore, it is important to develop robust and cost-effective imaging methods sensitive to the cellular composition of plaques.